Mechatronics 1

Filters & Regulators

2

Filters & Regulators

• Power Supply Review– transformer– rectifier– filter– regulator

• Rectifier Review– diode review– half wave operation– center tap full wave operation– bridge operation

• Filters– capacitor review– RC time constant review– capacitor filter operation– ripple voltage

• Regulators– IC regulators– line regulation– load regulation– zener diodes

3

Power Supply Overview

4

Power Supply Overview

Transformer – sets the appropriate voltage level

Rectifier – rectifies AC input voltage to pulsating DC voltage (can be half wave or full wave rectified

Filter – eliminates fluctuations in the rectified voltage and produces a relatively smooth AC voltage (this function was performed by the capacitor in the last part of Lab 5)

Regulator – maintains a constant voltage despite variations in the input line voltage or the load

5

Power Supply Overview

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Rectifier Review

•Diode review•Half wave operation•Center tap full wave operation•Bridge operation

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Diode Review

Key things to remember:• General

– P material is the anode– N material is the cathode

• Forward bias– the applied voltage is more negative at the cathode with

respect to the anode– if barrier voltage is overcome, current will flow– acts like a closed switch

• Reverse bias– the applied voltage is more negative at the anode with

respect to the cathode– no current will flow (unless breakdown voltage is achieved –

zener diode)– acts like an open switch

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Forward bias

When a pn junction is forward-biased, current is permitted. The bias voltage pushes conduction-band electrons in the n-region and holes in the p-region toward the junction where they combine.

The barrier potential in the depletion region must be overcome in order for the external source to cause current. For a silicon diode, this is about 0.7 V.

p-region n-region

p n

+

R

VBIAS

The forward-bias causes the depletion region to be narrow.

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Reverse bias

When a pn junction is reverse-biased, the bias voltage moves conduction-band electrons and holes away from the junction, so current is prevented.

The diode effectively acts as an insulator. A relatively few electrons manage to diffuse across the junction, creating only a tiny reverse current.

p-region n-region

p n

+VBIAS

R

The reverse-bias causes the depletion region to widen.

10

The P-N Junction

“The polarity of applied voltage which causes charge to flow through the diode is called Forward Bias.”

Source : http://www.st-and.ac.uk/~www_pa/Scots_Guide/info/comp/passive/diode/diode.htm

“The polarity of applied voltage which can't produce any current is called Reverse Bias.”

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Bias Recognition

Forward

Forward

Forward

Reverse

Forward

Reverse

Reverse

Forward

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Bias Recognition

Reverse

Reverse

Forward

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Half Wave Rectifier

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Average Voltage Value

• the average voltage is a measure of the efficiency of the rectifier circuit

• the “straight line” dc equivalent of the pulsating dc created by half wave rectification

• the value you would measure on a dc voltmeter

V p(out)

V AVG = ----------

π

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Center Tapped Full Wave Rectifier

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Bridge Full Wave Rectifier

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Average Voltage Value

• twice that of half wave rectified output 2V p(out)

V AVG = ----------

π

Since 2/ π = 0.637, you can calculate V AVG = 0.637 V p(out)

The full wave rectifier is twice as efficient as the half wave rectifier

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Filters

• Capacitor review• RC time constant review• Capacitor filter operation• Ripple voltage

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The Basic Capacitor

Capacitors are one of the fundamental passive components. In its most basic form, it is composed of two conductive plates separated by an insulating dielectric.

The ability to store charge is the definition of capacitance.

Dielectric

Conductors

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Die le c tric

Pla te sLe a d s

Ele c tro ns

BA

+

+

+

+

+

+

+

+

Initially uncharged

+ BA

+

+

+

+

+

+

+

Charging

+ BA

V S

+

+

+++++++++

Fully charged

BA

VS

+

+

+++++++++

Source removed

The charging process…

A capacitor with stored charge can act as a temporary battery.

The Basic Capacitor

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Charging

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Discharging

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How fast does a capacitor charge or discharge?

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The RC time constant

When a capacitor is charged through a series resistor and dc source, the charging curve is exponential.

C

R Iin itia l

t0(b ) C ha rg ing c urre nt

Vfina l

t0(a ) C a p a c ito r c ha rg ing vo lta g e

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Capacitor Charging Voltage Curve

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Tau (T)

•The voltage across a capacitor cannot change instantaneously because a finite time is required to move charge from one plate to another

•The rate at which the capacitor charges or discharges is determined by the RC time constant of the circuit

•The time constant of a series RC circuit is a time interval that equals the product of the resistance and capacitance

T = RC

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When a capacitor is discharged through a resistor, the discharge curve is also an exponential. (Note that the current is negative.)

t

t

Iin itia l

0

( b ) D is c h a rg in g c u rre n t

V in itia l

0( a ) C a p a c i t o r d is c h a r g in g v o l t a g e

C

R

The RC time constant

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Capacitor Discharging Voltage Curve

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Universal exponential curves

Specific values for current and voltage can be read from a universal curve. For an RC circuit, the time constant is

τ RC

100%

80%

60%

40%

20%

00 1t 2t 3t 4t 5t

99%98%

95%

86%

63%

37%

14%

5% 2% 1%

Number of time constants

Per

cent

of f

inal

val

ue

Rising exponential

Falling exponential

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Half Wave Rectifier with Capacitor Filter

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First Quarter Cycle

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Remainder of Cycle

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Second Cycle

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35

Ripple Voltage

• the variation in the output voltage•much improved when you add filtering• the smaller the ripple, the better the filtering and

the better quality dc output

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Half Wave & Full Wave Ripple Comparison

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Ripple Voltage

•Make RC > 10T•The ripple factor (r) is an indication of the

effectiveness of the filter and is defined as the ratio of the ripple voltage (Vr) to the dc (average) value of the filter output voltage (VDC)

r = (Vr / VDC) x 100%

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Regulators

• IC regulators• Line regulation• Load regulation

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IC Regulators

•Filters reduce ripple from a power supply to a relatively low level (<10%)

• Integrated circuit regulators connect to the output of a filtered regulator and reduce the ripple to a negligible level

•Regulators maintain a constant output voltage despite changes in the input voltage, load current or temperature

•Available in a variety of voltages

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A Basic Regulated Power Supply

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Percent Regulation

Regulation as a percentage is a figure of merit used to measure performance of a voltage regulator

Line RegulationHow much change occurs in the output voltage for a given change in the input voltageLine Regulation = (ΔVOUT / ΔVIN)100%

Load RegulationHow much change occurs in the output voltage from no load to full loadLoad Regulation = (VNL – VFL / VFL)100%

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Measures

Efficiency – Average Voltage Value (VAVG)

Filter Quality – Ripple Voltage (Vr) & Ripple Factor (r)

Regulator Quality – Line & Load Regulation